As a conventionally-known water vapor observing apparatus, for example, Patent Document 1 discloses in claim 4 a water vapor observing apparatus including a noise measuring means for measuring, according to transmission waves discharged to a weather subject at two frequencies with different attenuation amounts or phase shift amounts caused by water vapor, levels L1 and L2 or phases φ1 and φ2 of reflection waves individually arriving from the weather subject to an antenna system of a weather radar, and a converting means for converting a difference between the levels (=L1−L2) or a difference between the phases (φ1−φ2) into an amount or a distribution of water vapor in the direction of the weather subject. According to this water vapor observing apparatus, the distribution of water vapor can be obtained based on the reflection wave arrived from the weather subject. Note that, in Patent Document 1, rain cloud is exemplified as the weather subject.